by Ali Mackisack
She’s been spending her days looking at tiny pictures of sounds. The sounds reveal themselves as lines of varying lengths and shades, yet many many times, the picture frames on her computer screen are simply blank.
But when a sound picture appears, it’s like discovering treasure amongst the silence, and the rush of adrenaline is very real. There it is – a message from the bush at night! The picture itself says “a bat was here.” The lines show the sounds it was making and what it was doing. The picture is a spectrogram. The person peering at the screen is Julia from Sustainable Wairarapa Inc. And the bat itself? Well, it could be one from the bush near you!
Sustainable Wairarapa Inc (SWI) have been tracking long-tailed bats for many years alongside their many other conservation-based projects in the Wairarapa. Now, with a wave of interest in these tiny endemic creatures, SWI are fielding an increasing number of calls from people who think they might have bats living nearby.
Long-tailed bats fly at dusk along the edges of bush, and sometimes people think they have caught a glimpse of them as they fly around to feed. Other people plan to cut down big old trees and want to check first that they aren’t destroying one of the many roosts which long-tailed bats may occupy sporadically for a few days at a time. Tiny short-tailed bats, with their tendency to feed on the forest floor and live in groups in the cavities of old native trees, are desperately vulnerable and much trickier to find.
Technology has made some things easier, and has certainly made detecting bat calls easier. The set of bat detectors used by SWI, which were bought with a grant from the Department of Conservation (DOC) Community Fund, can be left hanging out in a likely location for up to a week at a time, and will usually only record when a bat call is heard. However, other ultrasonic bush noises in the same range at which the units are set to record, can also trigger a recording. So some of the recordings will show not-so-exciting recordings such as raindrops, wind or insect noises.
Julia loves downloading the soundcards after the detectors have been out on location, and the thrill of confirming bats are still active in places which they’ve been previously detected, or recorded for the first time in a new location. Sometimes however, the spectrograms are sadly “silent,” such as after a recent survey at the Kiriwhakapapa Campsite.
“It seems to be a likely spot for bat activity,” says Julia. “They love a wide open area such as the grassy campsite, surrounded by established native bush and a river. But no bat passes were detected. We placed the monitors along the edges of the grassy campsite, a short way up into the redwoods and along the stream.”
“This doesn’t mean that there aren’t bats in the wider area, but they didn’t visit our units.”
“Last year a lot of our monitoring was like this – places that seemed likely candidates didn’t have any bats. But the first four operations in February and early March this year were either back at sites where we’d previously found bats, or where we expected from other data that there were bats – and they all found lots of activity this time round. So it started to feel normal to find activity on our recordings. It’s quite good to be reminded that it is still special to detect long-tailed bat activity!”
If you want to be part of finding that “still special” activity, there are a number of ways to be involved. If you’re a boot-on-the-ground kind of person, the small team is always keen to welcome people who want to walk with them in some beautiful places and hang-out or collect-in the acoustic monitoring units. Some of these walks, such as into the river gully of Rewa Bush out towards the coast, are a treat for the intrepid. Other trips, such as the recent visit to Kiriwhakapapa Road end, are a lovely stroll.
And there’s a place too for those who would just love to know what’s going on. Julia has produced some interesting graphs that show things like patterns of activity in monitored locations, what a night “looks” like in a certain spot, and a map that shows where bats have been found in our region’s big backyard. The group hopes to obtain more acoustic recorders and make them available to members of the public to put out on their own property, which would add to this local body of information.
“At the moment, all our data just gets sent to the national bat database, and it’s really only researchers that look at that,” says Julia. “We’d love it if there was some local interest and a group of people who’d like to be in the loop about what we’re finding and where we might like to look.”
If you’d like to be in that loop, would like to go out monitoring, or think that there might be bats near you, you can contact the small team of those involved at firstname.lastname@example.org
- The long-tailed bat (Chalinolobus tuberculatus) and the short-tailed bat (Mystacina tuberculata) are endemic to New Zealand Aotearoa. They are a type of micro-bat – they’re tiny and weigh only a few grams. Most micro-bat species echo-locate almost constantly as they fly.
- Most long-tailed bat calls are at a frequency of 40 kilohertz (kHz).
- These ultrasonic sounds are no different in their physical properties than ‘normal” sounds. It’s just that humans can’t hear the sounds at this frequency. The upper limit of the audible range for humans is about 20 kHz for a young healthy adult.
- A spectrogram is a “picture” of sounds, showing how it varies with time. The time goes across the bottom (x-axis) and the frequency (in kilohertz) is shown on the vertical axis.
- A spectrogram can show what a bat was doing at the time that its calls were recorded. When a foraging bat is in a search phase, its calls are quite far apart. The pulses get closer together and use a wider range of frequencies as the bat approaches its target. Just before it captures its prey, the pulses of the calls are so fast they appear as a blur on the spectrograms. This is called a “feeding buzz.”A ‘bat pass” is a train of echolocation pulses recorded on a spectrogram.
- Some long-tailed bats have recently been found roosting in a farm building in Waipawa. While this is unusual, it’s thought that this group has made this adaptation because there’s no other options in terms of big old trees around in which to roost.
- New Zealand Aotearoa has its very own batman! Bat enthusiast Ben Paris, regularly posts sightings, snippets, opportunities and information on Facebook at NZ Batman.
Some of this additional information has been taken from Bat Call Identification Manual for DOC’s Spectral Bat Detectors 2017, Dr Brian Lloyd for the Department of Conservation.